Cementiferous paints



United States Patent 'CEMENTIFEROUS PAINTS John-Edwin Oakley Mayne, Cambridge, Englainl, asslgnor to The British 'Iron and Steel Research Association, London, England, a British company vN! .Dl wing. Application January .6, 195.4 S ria N 40 91 Claims priority, application Great Britain November 11, 1949 .11 Claims. (0.10.6914) 2,874,057 Patented Feb. 17, 1959 rusty surfaces :a stronger solution up to 35% is necessary, the ratio of zinc dust to solid phosphate being approximately '12.'9:'1 (since 10 ccs. of this stronger solution contains about .42. gms. of phosphate). The solution has-a pH value in the region of 9.9.

Example III In this composition the phosphate solution is a mixed solution of saturated potassium dihydrogen phosphate and a,3. 0% aqueous solution of dipotassium hydrogen phosphate, the pH value being 8 or slightly more than :8.

=' The proportions of zinc dust :andphosphate solution are as in the previous examples.

The present invention also contemplates the use of sodium phosphates but the potassium phosphates used in the above-compositions cannot in Practice be wholly re- 1 Pl ced-by the corresponding sodium salts as the solubility faces to produce thereon a firmly adherentcoating pen I taining sufficient metallic zinc to afior'd cathodic protection to the iron or steel. i Another object of the invention is to provide a cementiferou's paint: which, although setting rapidly after'application to a surface, will have a satisfactorily long pot life.

A more specific object oi the invention is to provide a rapid setting, cementiferous paint, having a satisfactorily long pot life, which consists essentially of an aqueous solution of an alkali metal phosphate andsuch a quantity of metallic zinc .dust that .upon application of a film of the paint to a surface the alkali metal phosphate will react with a part only of the zincto form a zinc phosphate cement, ,the remainder of the zinc constituting a cathodic protecting layer, firmly 'bound'to the .surfaceby the zinc phosphate cement. Various examples of compositions of'paint in accord-' ance with the invention will now be given:

Example I mixed with a 20% aqueous solution of dipotassium "hydrogen phosphate (-pH"8.'5 in the following' -proportions-z Phosphate solution 9s,, .4.,cus 10 The density of 20% solution of JQHPO; at normal temperatures is 1.2 gms/cci so that 10 ccs. of the solution contain 2.4 gms. of solid phosphate. 'Thus. the ratio of zinc dust-to. solid phosphate in the mi); is 22.5 to

Such a rnixture forms a paint suitable for use on clean metal surfaces, such as steel. However for satisfactory adhesion to rusty surfaces it is desi able to use a stronger phophate solution of up to 10 .ecs,.,of which contain 3.6 gins; of solid phosphate, so that in this case the ratio zinc dust/solid phosphate is 15:1 The need. tor a stronger solution is thought to be dueito the fact that the rust absorbs some of the solution and thereby reduces the quantity of the phosphate avail-able for reaction with the zinc dust. I i

- Example ll I-n this composition the 'dipotassium hydrogen phosphate solution of Example I-is replaced'b'y-a 25% aqueous solution of tripotassium phosphate, the proportions of zinc dust and phosphate solution being the same. For use on of the latter at normal temperatures is too low to ensure satisfacto y cemen f rmation.

Four compositions utilising mixtures of sodium and no assiumph snhat s are as f llows: e

T i Example IV g I The phosphate in the solution .of Example Lis replaced by a mixture of disodi in hyd ogen p phate and .di-

potas ium. hydr g n ph sph e n h proportion 5 g ns.

to 1.8 .gmsrespectively, this ratio giving a value of about. 8.5.,

Example, V

The phosphate i'n the solution of Example I is replaced bya mixture of sodium dihydrogen phosphate and tr'ipotassium phosphate in the proportion 9 g ns. to 15 gms. respectively this ratio giving .a-pH value of about 8.5. 7

Example V1 The phosphate in the solution of EXample'I is replaced by a mixture-of trisodiumphosphate and potassium dihydrogen Phosphate in the proportion 3 gms. toh6 gins. respectively, this ratio again giving a pH value of about "Example VII The phosphate in the solution of "Example 1 is replaced by a mixture of tr is'odium phosphate and tripotassium phasphate in any possible proportion {the amount of the sodium salt present being {limited by its-solubility). The- Zinc dust free from lumps and containing about 4% of zinc oxide, which is always present in zinc dust, is

resulting solution has a'pH value of about 9.9. I

- In 'all the above examples, the pH of the solution is not less than eight, and in most instancesis 8.5 or even higher. Thisisa-n important feature of the "invention since more acid so'lutions cause the mixture to set too rapidlyfor its use as a paint to be practicable;

The solutions preferably contain from about-2.4 grams to about 4.2 grams of-phosphate per each 1 -O ecs., and

the phosphate concentration =of1the solutions is from about 205 0 about The proportion of zinc dust to solid phosphate, is not of] course attected by dilutionwith water. In practice he amount wa e ed to he so u o illdepend on h me hodfby which the pa n i o be applied; thus, 50

to. .6. .u tsby weigh of. zin t par by olume of solution (as in all t a e e amp e :is about theoptimum ratio for brushing. It, on the other hand, the paint is to be applied by spraying or dipping, more water should b add dt It: will be clearly understood that other ingredients normally presentin paint, such aspigments and wetting agents, may be included in the compositions described in tho.examples..-..Moreoner substances may beiincorpbrated which are selected in order to discourage the growth of micro-organisms on surfaces treated. Such substances are, for example, compounds of copper, mercury or arsenic or organic compounds.

The new paints are easy and cheap to manufacture, since the ingredients only require mixing, provided'that the zinc dust is free from lumps. They remain. brushable for a considerable time after mixing and have been stored in covered containers at room temperature for periods of more than three days. If made up with too strong a phosphate solution, or stored for too long at too high a temperature, they become difficult to apply, although this effect of storage can be overcome by the addition of water. 1

In practice, however, it is thought preferable always to use freshly mixed paint.

When any of the paints of the above examples is well brushed out on clean or rusty steel surfaces the weight of the dried film obtained is from 0.4 to 0.8 ounce per square foot, equivalent to a coverage of 20 to 40 square feet per pound. Satisfactory coatings as thick as 1.6 ounces per square foot can be obtained by liberal application of the paint.

The paints set within an hour of application and their setting is not appreciably affected by the relative humidity of the atmosphere. They can therefore be applied in windy situations and in very high humidities such as those which prevail in a dry dock.

While the invention is not limited by any particular theory as to the mode of action of the new cementiferous paints, it appears likely that the potassium phosphate solution first reacts with the zinc oxide which is always present in zinc dust, with the formation of zinc phosphate, potassium hydroxide and water. This reaction can only proceedfor a short time owing to the limited supply of zinc oxide; it is therefore probably succeeded by a reaction between the Zinc dust and potassium phosphate, which forms Zn (PO .4H O or 3 [Zn (PO ;Zn(OH) potassium hydroxide and hydrogen.

If the setting reaction is assumed to be:

then for cement formation it will be seen that'65 gms. zinc react with 141 gms. phosphate. From these figures, the use in the mix of equal quantities of zinc dust and phosphate will certainly leave some excess of zinc for the cathodic protection of the steel; in practice, however the weight of zinc to dust used is more than eleven times the weight of the phosphate. In any event the zinc phos-v phate ratio should not exceed 25 in the case of dipotassium phosphate, and 20 in the case of the tribasic salt.

The reaction set out above is slow when the paint is stored in bulk, probably due to the fact that the hydrogen is not readily removed from the surface of the zinc; when, however, the paint is spread out in athin film, the zinc is depolarised by atmospheric oxygen and rapid, setting of the paint ensues. Secondary reactions may take place between the zinc dust and the potassium hydroxide, with the formation of complex zincates.

The dried films of phosphate paints are alkaline to phenolp'hthalein paper, but this alkalinity does not afiect the drying of linseed oil applied over the paint film. The paints in accordance with the invention are water stable and are'suitable for use as priming coats for the protection of'metal against immersion in sea-water. Other possible applications of the new paints are their use. as a wash on pickled steel to prevent the formation of rust before paint is applied and their use for the prevention of stress corrosion in aluminum alloys.

I claim:

1. A cementiferous paint for the protection of ferrous metal surfaces consisting essentially of a mixture of zinc dust and an aqueous solution containing at least about 20% of an alkali metal phosphate of the group consisting of, monopotassium dihydrogen phosphate, dipotasi of solution.

sium hydrogen phosphate, tripotassium phosphate, a mixture of potassium phosphates, and a mixture of a potassium phosphate and a sodium phosphate, said aqueous solution having a pH value within the range from about 8 to about 9.9 and the weight ratio of zinc dust to said alkali metal phosphate being within the range from 1:1 to 25:1.

2. A cementiferous paint composition as defined in clairnl in which the pH value of the phosphate solution is about 8.5.

3. A cementiferous paint composition as defined in claim 1 in which the aqueous phosphate solution contains between 2.4 gms. and 4.2'gms. of phosphate in each 10 cc.

4. A cementiferous paint composition as defined in claim 1 in which the concentration of the aqueous phosphate solution is within the range from 20% to by weight. w

5. A cementiferous paint composition as defined in claim 1 in which the phosphate consists of dipotassium hydrogen phosphate. I

6. A cementiferous paint composition as defined in claim 1 in which the phosphate consists of tri-potassium 9; A cementiferous paint' composition asdefined in claim 1 in which the phosphate consists of a mixture of sodium dihydrogen phosphate and tri-potassium phosphate.

10. A cementiferous paint for the protection of metal surfaces consisting essentially of an admixture of zinc dust containing about 4% of zinc oxide with an aqueous solution containing at least 20% of dipotassium hydrogen phosphate, said solution having a pH of about 8.5, and the Weight of the zinc dust being at least equal to and not more than 25 times as great as the weight of phosphate contained in said solution, the Zinc dust and the dipotassium hydrogen'phosphate constituting at least the major solid ingredients of the paint.

11. A cementiferous paint for the protection of metal surfaces consisting essentially of an admixture of zinc dust containing about 4% of zinc oxide with an aqueous solution containing at least 20% of tri-potassium phosphate, said solution having a pH of about 9.9, and the weight of the zinc dust being at least equal to and not more than 20 times as great'as the weight of phosphatecontained insaid solution, the zinc dust and the tri-potassium phosphate constituting at least the major solid ingredients of the paint. a

I References Cited in the file of this patent -UNITED STATES PATENTS Great Britain May 14, 1947 

1. A CEMENTIFEROUS PAINT FOR THE PROTECTION OF FERROUS METAL SURFACES CONSISTING ESSENTIALLY OF A MIXTURE OF ZINC DUST AND AN AQUEOUS SOLUTION CONTAINING AT LEAST ABOUT 20% OF AN ALKALI METAL PHOSPHATE OF THE GROUP CONSISTING OF, MONOPOTASSIUM DIHYDROGEN PHOSPHATE, DIPOTAS SIUM HYDROGEN PHOSPHATE, TRIPOTASSIUM PHOSPHATES, AND A MIXTURE OF POTASSIUM PHOSPHATES, AND A MIXTURE OF A POTAS SIUM PHOSPATHE AND A SODIUM PHOSPHATE, SAID AQUEOUS SOLUTION HAVING A PH VALUE WITHIN THE RANGE FROM ABOUT 8 TO ABOUT 9.9 AND THE WEIGHT RATION OF ZINC DUST TO SAID ALKALI METAL PHOSPHATE BEING WITHIN THE RANGE FROM 1:1 TO 25:1. 